1. Field of the Invention
The present invention relates to an object detector, for example, photo-interrupter in which detection of an object is carried out based on whether or not a light emitting element and a photodetector are optically coupled with each other.
2. Description of the Related Art
A photo-interrupter is a well-known example of object detectors of the above-mentioned type. Of various types of interrupters, two-terminal types are receiving much attention because of their low mounting cost.
The above-mentioned two-terminal type photo-interrupter basically includes a light emitting element, a photodetector, an amplifying circuit, a switching element, and a resistor. For example, a circuit in which a light emitting diode (LED), photo-diode, and NPN transistor are respectively employed as the light emitting element, photodetector, and switching element has the following structure.
The anode of the LED is connected to the first terminal (power terminal) to which a power supply voltage V.sub.CC is applied, and the resistor is connected between the cathode of the LED and the second terminal (output terminal). The collector of the above-mentioned NPN transistor is connected to the cathode of the LED, and the emitter thereof is also connected to the second terminal. The photo-diode is arranged so as to receive the light emitted from the LED, or more specifically the photo-diode is optically coupled with the LED. The anode of the photo-diode is connected to the second terminal, and the cathode thereof is connected to the input terminal of the above-mentioned amplifying circuit. Further, the first and second power supply terminals of the amplifying circuit are respectively connected to the first and second terminals so as to supply driving power thereto, and the output terminal of the amplifying circuit is connected to the base of the above-mentioned NPN transistor. The LED, photo-diode, amplifying circuit, NPN transistor, and resistor are sealed in the same package, from which the first and second terminals are lead out to serve as external terminals.
The following are descriptions of the usage and operation of the two-terminal type photo-interrupter.
To start using this photo-interrupter, a load resistor (external resistor) is connected between the second terminal and a ground GND located outside the interrupter, and a power supply voltage V.sub.CC is applied to the first terminal. Accordingly, the LED emits light when a driving current is supplied thereto. Then, the photo-diode carries out photo-electric conversion upon reception of light from the LED to output a current signal. The current signal thus output is amplified by the amplifying circuit, and the NPN transistor is switched ON/OFF by the amplified output thereof.
In the case where there is no object in the optical path from the LED to the photo-diode, (where light is detected), a current signal, the level of which corresponds to the amount of light received from the photo-diode, is output to the amplifying circuit, where the signal is amplified, and the amplified signal is supplied to the base of the NPN transistor. As a sufficient base current is supplied to the base of the NPN transistor, the transistor is turned on, and saturated between the collector and emitter. Where the saturated voltage between the collector and emitter, and the forward direction voltage of the LED are represented by V.sub.CE (sat), and V.sub.F, respectively, the second terminal's output voltage V.sub.0 is expressed as: EQU V.sub.0 =V.sub.CC -V.sub.F -V.sub.CE (sat) (1)
On the other hand, in the case where there is an object on the optical path from the LED to the photo-diode (where light transmitting from the LED to the photo-diode is obstructed by the object, and the light is not detected by the photo-diode), a current signal is not output from the photo-diode, thereby turning off the NPN transistor. Consequently, a current I.sub.F flowing into the LED runs out to the ground GND via the resistor and the load resistor. The output voltage V.sub.0 in this case can be expressed as: EQU V.sub.0 =(V.sub.CC -V.sub.F)/(1+R.sub.l /R.sub.L) (2)
where the resistance value of the above resistor, and that of the load resistance are represented by R.sub.l and R.sub.L, respectively. Here, if each of the resistance values is selected so as to satisfy R.sub.l &gt;R.sub.L , then the V.sub.0 value obtained by equation (1) will be larger than that obtained by equation (2). This tells us that the output voltage V.sub.0 in the case where there is an object between the LED and the photo-diode, differs to that in the case where there is no object. Taking advantage of this fact, whether or not there is an object between the LED and photo-diode can be detected from the change in the second terminal's output voltage V.sub.0.
However, with the above-mentioned two-terminal type photo-interrupter, the range of power supply voltage V.sub.CC which can be used is very much limited, for the following reason.
In the case where an object exists between the LED and the photo-diode, the output voltage V.sub.0 is expressed by the equation (2). In this case, a current I.sub.F flowing through the LED can be expressed as: EQU I.sub.F =(V.sub.CC -V.sub.F)/(R.sub.l +R.sub.L) (3)
The forward direction voltage V.sub.F of the LED is substantially constant; however if the range of power supply voltage V.sub.CC used is wide, the current I.sub.F flowing through the LED greatly changes. To be more specific in terms of values, when V.sub.F =2 V, R.sub.l =1 k.OMEGA., and R.sub.L =300 .OMEGA., the following can be obtained from the equation (3).
When V.sub.CC =5 V, I.sub.F =2.31 mA, and PA1 when V.sub.CC =24 V, I.sub.F =16.9 mA.
Thus, when the power supply voltage V.sub.CC increase about five-fold, the current I.sub.F varies about seven-fold.
As described, when the power supply voltage used is high, the current I.sub.F becomes still higher, putting too much load on the LED. As a result, the characteristics of the LED are likely to deteriorate. On the other hand, when the power supply voltage is low, the current I.sub.F becomes still lower, weakening the optical coupling between the LED and the photo-diode. As a result, detection of an object cannot be effectively performed.